Developments in combustion engine technology have shown that compression ignition engines, known as diesel-cycle engines, may be fueled by gaseous fuels without sacrifices in performance or efficiency. Examples of such fuels include natural gas, methane, propane, ethane, gaseous combustible hydrocarbon derivatives such as methanol and hydrogen. Substituting diesel with such gaseous fuels generally results in cost, availability and emissions benefits over diesel. These developments, however, require such gaseous fuels to be delivered to the engine for combustion at high pressures.
Such prior art high pressure gas delivery systems, however, have been burdened by challenges arising from the need to provide a practical gas fueling system that supplies adequate on-board fuel storage while, at the same time, ensuring that the platform integrating the power generation system, be it stationary power or vehicular power, is not unduly burdened by additional equipment and/or large fuel tanks. The present invention allows, amongst other things, for a fuel delivery system that helps to:    minimize the space required for such a system;    maximize the operating time or range of such gas powered vehicles; and,    deliver a gas at the required operational pressures.
Natural gas and other gaseous fuels can be stored in tanks either as compressed gas (CNG in the case of natural gas), or cryogenically as a liquid (LNG in the case of liquefied natural gas). The present invention is directed to a method and apparatus that utilizes cryogenic storage. By way of example, the energy density of LNG, depending on its comparative pressure and temperature, is approximately three times that of CNG, thereby providing a significant storage advantage over CNG systems. Natural gas stored as LNG allows for more fuel to be stored per unit volume.
Cryogenic liquids are liquids that boil at temperatures below approximately 200K. Such gases include, by way of example, natural gas, nitrogen, methane, hydrogen, helium and oxygen. While, as mentioned above, there are advantages to utilizing LNG and other liquefied gases, cryogenic storage presents its own challenges.
The goal of a fuel delivery system based on cryogenic fuel, is to provide a warm pressurized gaseous fuel to a fuel injector from a cold liquefied store of such fuel. Some prior art systems have accomplished this by pumping cold liquid fuel from a cryogenic tank utilizing a pump physically separate from the tank so as not to burden the cryogenic environment with a heat leak source. The pump elevates the pressure of the fuel and delivers it to a heater where the fuel is heated to a pre-determined temperature suitable for use as a gaseous fuel. Further, where occasions arise in which a pressurized fuel is required to meet a sudden demand that cannot be immediately met by the pump alone, an accumulator may follow the heater thus allowing for a ready supply of fuel to be stored at or near the approximate conditions required for injection as a gaseous fuel into a combustion engine.
One potential goal of utilizing a gaseous fuel is to replace diesel fuel. However, in light of the delivery system described above, a gaseous fuel delivery system would require three more physically separate components than is the case for a similar diesel fuel delivery system, namely, a physically removed pump, accumulator and heater. Moreover, numerous fittings and connectors are required to join together such a fuel system each of which is a potential failure point or leak path compromising the reliability of such a system as a whole.
The subject invention significantly reduces:    the space required for such a fuel system;    the material costs associated with building this system;    the potential failure points within the gas fuel system; and,    the exposed cryogenic components of the fuel delivery system.
One way of dealing with the space and reliability issues arising with a fuel delivery system similar to the one described above is to incorporate a pump or an equivalent pressurizing system into the cryogenic tank. Prior art delivery systems have contemplated such pumps. For example see U.S. Pat. Nos. 4,472,946 and 5,327,730.
A concern with introducing a pump directly into the cryogenic tank is that it may create a potential heat leak thereby reducing the holding time of the liquefied gas, that is, the time prior to which the relief pressure valve opens to vent gas so as to avoid excessive pressures within the tank. Moreover, some prior art fuel delivery systems utilize in-tank centrifugal pumps and vaporizers. Centrifugal pumps, however, work best where relatively low pressure gas must be provided. In diesel-cycle engines, the high pressure direct injection of gaseous fuels requires pressures far in excess of those that can be practically provided by centrifugal cryogenic in-tank pumping systems. Pumping systems utilizing a centrifugal pump are appropriate for transfer pumps and fueling station operations.
A similar problem arises where heating systems are used to provide pressurized gas. Such systems boil gas within the cryogenic tank and release it from its liquefied state in this fuel delivery system at between 15 and 125 psig (103 to 861 kPa). These systems are also unsuitable for high pressure direct injection engines where improved efficiency and emissions can be achieved.
The discussion in this application generally considers a system that provides a pressurized gas from a liquefied store of that gas. However, for the purposes of this application, it will be understood that any references to fluids include liquids as well as liquids pressurized above the supercritical point of the gas of interest. Similarly, any references to gases include gases as they are generally defined as well as gases pressurized above the supercritical point of those gases. More generally, if the desired substance to be delivered is to be delivered at a pressure placing it above the supercritical point of the substance, then that substance generally will also be included in any reference to a gas where corresponding fluid is, at some point in the gas delivery system, at a lower temperature and pressure prior to being delivered.